Lift-type valve assemblies are well-known in the field of fluid moving structures and a variety of different designs have been developed in an attempt to achieve efficient and reliable control of fluid movement. Such valve assemblies typically have valve members which are retained by springs in a closed position in separate inlet and outlet passages and prevent fluid flow therebetween. The valve members are automatically movable to open positions when the inlet passage fluid pressure exceeds the outlet passage fluid pressure by a sufficient amount. This fluid pressure "differential" is a function of several factors including the closing force exerted by the springs; the amount of surface area exposed to fluid pressure on either side of the valve member; and the amount of cohesion or "stiction" between the valve member and valve seat. In fluid moving structures such as pumps and compressors reducing the fluid pressure differential required to open the valves will generally result in an increase in operating efficiency, because less "work" is required to move an equivalent amount of fluid and because lower fluid temperatures will be maintained at such lower pressures.
In operation, such valve assemblies automatically open and close very rapidly and the valve members strike the valve seats with considerable force. Therefore, the useful life of the valve members may be prolonged by providing a substantial contact area between the valve members and the valve seats to absorb the shock upon engagement as the valve closes. However, such a substantial contact area can be detrimental to valve efficiency because substantially less surface area of the valve member is thereby exposed to the greater inlet passage fluid pressure than is exposed to the outlet passage fluid pressure. Because the lifting force is a function of both the fluid pressure and the surface areas upon which such pressures act, increasing the contact area generally increases the required pressure differential. Also, further operating efficiency is lost because of the cohesion or "stiction" present thereat which tends to resist separation and opening of the valve assembly. Such a cohesion may be caused by a vacuum effect or a molecular attraction or both between the mating surfaces and is common in lift-type valve assemblies where such mating surfaces tend to be relatively close fitting to minimize fluid leakage. Therefore, a greater fluid pressure differential is required to overcome such cohesive force and fluid moving structure efficiency is accordingly lessened.
Prior art valve assemblies utilizing substantial contact areas for increased valve life have heretofore not provided relief means for alleviating such problems associated with substantial contact areas between the valve members and valve seats.